Cells have a checkpoint mechanism of such that, when the DNA therein is damaged, then the cells temporarily stop the cell cycle and repair the damaged DNA (Cell Proliferation, Vol. 33, pp. 261-274). In about a half of human cancers, a cancer-suppressor gene, p53 is mutated or depleted and the cells thereby have lost the G1 checkpoint function thereof. However, such cancer cells still keep the G2 checkpoint function remaining therein, which is considered to be one factor of lowering the sensitivity of the cells to DNA-active anticancer agents and to radiations.
A Weel kinase is a tyrosine kinase that participates in the G2 checkpoint of a cell cycle. Weel phosphorylates Cdc2(Cdk1) tyrosine 15 that participates in the progress to the M stage from the G2 stage in a cell cycle, thereby inactivating Cdc2 and temporarily stopping the cell cycle at the G2 stage (The EMBO Journal, Vol. 12, pp. 75-85). Accordingly, in cancer cells having lost the p53 function therein, it is considered that the G2 checkpoint function by Weel is important for repairing the damaged DNA so as to evade the cell death. Heretofore, it has been reported that the Weel expression reduction by RNA interference or the Weel inhibition by compounds may increase the sensitivity of cancer cells to adriamycin, X ray and gamma ray (Cancer Biology & Therapy, Vol. 3, pp. 305-313; Cancer Research, Vol. 61, pp. 8211-8217). From the above, it is considered that a Weel inhibitor may inhibit the G2 checkpoint function of p53-depleted cancer cells, thereby enhancing the sensitivity of the cells to DNA-active anticancer agents and to radiations.
As a low-molecular Weel kinase inhibitor, for example, known are compounds described in US Application 2005/0250836, WO2003/091255, Cancer Research, Vol. 61, pp. 8211-8217, or Bioorg & Med. Chem. Lett., Vol. 15, pp. 1931-1935. However, the compounds described in these references quite differ from the compounds of the invention in point of their structures.
On the other hand, WO2004/056786 or WO2005/021532 or WO2006/091737 disclose various compounds such as dihydropyrazolopyridines that are relatively similar to the compounds of the invention in point of their skeletons. However, these references do neither concretely disclose nor suggest any Weel kinase-inhibitory effect of those compounds as well as the compounds of the invention.